This invention relates to substantially closed-cell rigid foams containing urethane, urea and biuret groups which are suitable for use as insulating materials and/or for reinforcing mechanical structural components and to a process for the production of these foams.
Processes for the production of substantially closed-cell rigid foams containing urethane, urea and biuret groups are generally known.
A summary of processes for the production of rigid foams can be found, for example, in Kunststoff-Handbuch, Vol. VII, "Polyurethane", edited by R. Vieweg and A. Hochtlen, pages 504 et seq (Carl-Hanser-Verlag, Muchen 1966).
Rigid substantially closed cell foams are normally produced by reaction of aromatic polyisocyanates with a relatively high molecular weight polyol, preferably a polyether or a polyester polyol, in the presence of a blowing agent, a catalyst, a flameproofing agent and other auxiliaries and additives. Mixtures of polyether or polyester polyols have also been used instead of individual polyether or polyester polyols. Such mixtures have been used to reduce costs. More specifically, mixing expensive polyester polyols suitable for the production of high-quality rigid polyurethane (PU) foams with inexpensive polyether polyols reduces the overall cost of polyol used to produce the foam. Suitable low-viscosity polyester polyols for the production of rigid PU foams having favorable mechanical properties are described, for example, in German Offenlegungsschrift 27 04 196.
The esters used may also be recycled products of relatively high molecular weight esters. In this case, too, the esters may also be used as relatively inexpensive components for reducing the cost of the mixtures. These inexpensive esters are obtained, for example, by glycolysis of high molecular weight dimethyl terephthalate (DMT) polymers or by processing of the distillation residues remaining behind in the production of DMT.
Use of polyester polyols as the relatively high molecular weight polyhydroxyl compound for the production of rigid foams containing urethane groups, may improve the flame resistance and reduce the brittleness of the product foam. However, such polyester polyols show unsatisfactory miscibility with the halogenated hydrocarbons typically used as blowing agents due to their relatively high polarity by comparison with polyether polyols such as polyoxypropylene polyols.
German Offenlegungsschrift 36 42 666 teaches that this miscibility problem is obviated by using polyester polyols having lateral alkyl groups.
Each of the above-mentioned processes employs chlorofluorocarbon (CFC) blowing agents to obtain desirable rigid foam characteristics, such as for example toughness, adhesion to surface layers and reduced flammability.
Rigid foams produced in accordance with known processes without CFC blowing agents show significant disadvantages, namely: brittleness, flammability and non-adhesion to surface layers, as to be seen from the comparison example 3.